As a method for recovering valuable metals such as nickel and cobalt from a low grade nickel oxide ore having a low nickel grade by wet smelting, for example, as described in Patent Document 1, a high pressure acid leaching method (HPAL method) has been performed in which sulfuric acid is added to an ore slurry, followed by leaching under high temperature and high pressure.
Herein, in a low grade nickel oxide ore, there are two kinds of ore: a limonite-type ore having a high iron grade and a low alkaline component grade such as magnesium and silica and a saprolite-type ore containing a large amount of alkaline component, and the limonite-type ore has been mainly used as a raw material for the HPAL method.
On the other hand, in the case of using the saprolite-type ore as raw material, sulfuric acid added in a leaching treatment and an alkaline component contained in the ore react with each other to form an alkali sulfate such as magnesium sulfate, and as a result, the consumed amount of acid tends to increase, which is economically disadvantageous. For this reason, a case where the saprolite-type ore is subjected to pressure leaching based on the HPAL method is limited only to some amounts in which the balance between the recovered amount of valuable metals and the consumed amount of sulfuric acid is achieved; however, in many cases, the amount thereof stays at only a small ore amount of the saprolite-type ore to be produced.
Hitherto, for example, normal-pressure leaching methods have been studied as a method for effectively using a saprolite-type ore. Specifically, the methods are to use a saprolite-type ore as a neutralizer for a free acid contained in a leachate to be obtained by the HPAL method, and for example, are disclosed in Patent Documents 2, 3, and the like. However, the main objects of these methods are merely to utilize the saprolite-type ore as a neutralizer for a free acid or a magnesium source, and in the case of the use of the saprolite-type ore as a neutralizer, the recovery rate of valuable metals such as nickel and cobalt contained in the saprolite-type ore is low. Thus, it is difficult to say that the saprolite-type ore is effectively utilized as raw material.
Further, in a method disclosed in Patent Document 4, there is disclosed a method of leaching nickel and cobalt through normal-pressure leaching by using a saprolite-type ore as raw material. Although a high recovery rate can be achieved, the time necessary for the normal-pressure leaching is 9.5 hours or longer, which is extremely long and thus this method has poor productivity efficiency.
In this regard, Patent Document 5 proposes a method for recovering valuable metals by supplying a leaching residue obtained by normal-pressure leaching to an HPAL pressure leaching treatment. Specifically, the method disclosed in Patent Document 5 is to treat the entire amount of a low grade nickel oxide ore by normal-pressure leaching and pressure leaching. However, at a reaction temperature of 95° C., which is the condition for the normal-pressure leaching, there are a further need for a retention time of 2 hours to 3 hours in a reaction tank, a need for expanding the scale of equipment, supplying a large amount of heat for heating and warming, and the like. Thus, the method is not efficient in terms of actual operations.
Furthermore, since the main object of normal-pressure leaching of Patent Document 5 is also to neutralize the free acid in the leachate to be obtained by pressure leaching, the magnesium leaching rate in the normal-pressure leaching is merely about 42% to 50%, and thus the amount of sulfuric acid consumed by magnesium in the pressure leaching is still large.
Further, there is a problem in that the leaching rate of valuable metals such as nickel and cobalt decreases as the concentration of magnesium contained in the pressure leachate increases. For this reason, there is a problem in that the saprolite-type ore cannot be effectively utilized as raw material, for example, some of the normal-pressure leaching residue is discharged to a system, a saprolite-type ore having a magnesium grade of a certain degree or more cannot be used for the normal-pressure leaching, and the like.
In the aforementioned Patent Document 5, as a countermeasure therefor, recycling of the solution discharged from the process which contains sodium is proposed. Specifically, the solution discharged from the process which contains sodium is used for a multistage rinsing liquid for a pressure leaching residue, a diluted solution of a flocculant that is added at the time of solid-liquid separation in a thickener or the like, generation of a low grade nickel oxide ore slurry, and the like so as to remove iron and aluminum, which are impurity components at the time of the pressure leaching, as natrojarosite and natroalunite respectively, and thus the amount of sulfuric acid consumed by these impurity components is intended to be reduced. However, there is a concern that double sulfates such as natrojarosite and natroalunite become scale in an autoclave. Further, the solution discharged from the process which contains sodium is a magnesium sulfate solution obtained after recovering the valuable metals and magnesium sulfate is condensed by circulation in the process system. For this reason, crystal precipitation may occur in the process beyond the saturation concentration, and there is also a concern that filtration failures, pipe blockages, and the like occur.
Due to the reasons described above, in the HPAL method, the saprolite-type ore has not yet been effectively used as raw material, and thus there is a demand for a method capable of efficiently recovering valuable metals such as nickel and cobalt from the saprolite-type ore.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2005-350766
Patent Document 2: Japanese Unexamined Patent Application, Publication No. S60-75536
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2007-77459
Patent Document 4: Japanese Unexamined Patent Application (Translation of PCT Application), Publication No. 2008-530356
Patent Document 5: Japanese Unexamined Patent Application, Publication No. H06-116660